Device for electrically contacting electrodes in high-temperature fuel cells

ABSTRACT

The invention relates to a device for electrically contacting electrodes in high-temperature fuel cells. The inventive device is characterized by an interconnector plate with openings and contact elements, which are located therein and which are provided for electrically contacting electrodes. The contact elements protrude from both sides of the interconnector plate at a predetermined height, and the openings are sealed in a gas-tight manner.

[0001] The invention relates to a device for electrically contactingelectrodes in high-temperature fuel cells.

[0002] A fuel cell comprises a cathode, an electrolyte and an anode. Thecathode is supplied with an oxidizing agent, for example, air and theanode is supplied with a fuel, for example hydrogen.

[0003] Different fuel cell types are known, for example, the SOFC fuelcell of the publication DE 44 30 958 C1 and the PEM fuel cell of thepublication DE 195 31 852 C1.

[0004] The SOFC fuel cell is also known as a high-temperature fuel cellsince its operating temperature can amount to up to 1000° C. At thecathode of a high-temperature fuel cell oxygen ions are formed in thepresence of the oxidizing agent. The oxygen ions traverse theelectrolyte and recombine on the anode side with the hydrogen arisingfrom the fuel to water. With the recombination, electrons are liberatedand thus electrical energy is generated.

[0005] A plurality of fuel cells as a rule are electrically connected toone another by connecting elements and are mechanically joined byinterconnectors with one another for producing greater electricaloutputs. An example of a connecting element is the bipolar plate. Bymeans of bipolar plates fuel cells are stacked one upon another andconnected electrically in series. This arrangement is known as a fuelcell stack. A fuel cell stack is comprised of the bipolar plates and theelectrode-electrolyte units.

[0006] Interconnectors generally also serve as gas distributorstructures apart from their electrical and mechanical characteristics.In the bipolar plates the gas distributor structures are realized byribs with electrode contacts which separate the gas passages to supplythe electrodes (DE 44 10 711 C1). Gas distributor structures ensure thatthe operating agents are uniformly distributed in the electrodecompartments (compartments in which the electrodes are found).Advantageously, the following problems can arise with fuel cells andfuel cell stacks:

[0007] Metallic bipolar plates with a high chromium content formconductive chromium oxide cover layers; in operation there is anevaporation of chromium resulting in ageing characteristics within thefuel cell.

[0008] Metallically bipolar plates with high aluminum content form A1₂O₃ cover layers which act detrimentally like an electrical insulator.

[0009] In a fuel cell stack, between the known rigid bipoplar plates andthe electrode-electrolyte units, poorly conducting contact points canarise. These result among others from fabrication tolerances in theproduction of bipolar plates or electrode-electrolyte units.

[0010] The object of the invention is, therefore, to provide a devicewhich ensures long-term stability of the electrical contacting of theelectrodes in high-temperature fuel cells. A method of producing such adevice is presented.

[0011] The objects are achieved through a device according to claim 1.It is characterized by an interconnector plate with openings and contactelements which are found therein for the electrical contacting of theelectrodes whereby the contact elements on both sides of theinterconnector plate project to a predetermined height and close theopenings gas tightly [hermetically]. This teaching ensures that thecurrent flow through the interconnector plate is ensured through thecontact elements. Supporting and current conducting functions are thusdecoupled and ensured through the interconnector plate or the contactelements. As a result one is largely free in the choice in the materialsfor the interconnector plate. It is advantageous to use high-temperatureresistance [refractor] material for the interconnector plate wherebyalso nonmetallic materials can be employed. As the contact elements, setor rivet heads, pins, wires and rivets or other shapes suitable for thepurpose of current conducting and especially easily deformable materialcan be used. The contact elements are such that may be so incorporatedin the interconnector plates that they close the openings in a gas-tightmanner. The heights to which the contact elements project out of theinterconnector plate as well as the spacings of the contact elementsfrom one another can predetermine the heights and widths of the gasdistributor structures. This results in a reduction in labor time andproduction cost.

[0012] According to a feature of the invention it is provided in claim 2that all contact elements project to an identical height from theinterconnector plate (claim 2). In this manner, the electrical contactis ensured by all of the contact elements.

[0013] Especially advantageously, the contact elements contain at least99.97% silver (claim 3). The contact elements are then comprised of puresilver. Silver affords the advantage that it has long term stabilityunder reducing and oxidizing conditions as a good current conductorsince it forms no dense corrosion products which can increase thecontact resistance. Other current conducting deformable materials canhowever also serve, especially noble metals, like for example, platinum(Pt), gold (Au) or palladium (Pd) as starting materials for the contactelements.

[0014] According to claim 4, a high-temperature fuel cell encompassessuch a device according to the invention. Because of the characteristicsof the contact elements, like for example high-temperature resistanceand oxidation resistance, long-term stable contacting of electrodes iseffected with a reduction in the contact resistances in thehigh-temperature fuel cell.

[0015] Especially advantageously, a high-temperature fuel cellencompasses between the contact elements and an anode as the electrodean elastic nickel mesh (claim 5). This nickel mesh serves as anadditional means for ensuring uniform electrical contact between theanode and the contact elements via the grid points of the mesh and thusserves to compensate for the above-mentioned disadvantageous fabricationtolerances.

[0016] In a further embodiment of a high-temperature fuel cell, thelatter encompasses between the contact element and a cathode as theelectrode, an elastic silver mesh (claim 6). This serves as anadditional means for ensuring uniform electrical contact between thecathode and the contact elements through the grid points of the mesh andthus to compensate for poor transverse conductivity of the cathode. Bythe use of silver mesh, the number of contact elements can besignificantly reduced.

[0017] The fuel cell stack encompasses at least two suchhigh-temperature fuel cells (claim 7). In this manner higher outputs areproduced.

[0018] The objects are further achieved through a method of making sucha device (claim 8). It is thus provided that an opening is formed in aninterconnector plate having the area of the contact element and contactelements are form-fittingly introduced into the opening. Then thecontact elements are connected with the interconnector plate so that theopenings are closed in a gas-tight manner.

[0019] The gas-tight connection can be brought about for example byupsetting the contact elements. The upsetting operation can be effectedat room temperature. If a uni-axial press is used, the upsetting of thecontact elements can be carried out uniformly in that all of the contactelements project at identical heights from the interconnector plate.Through the upsetting of the material and the thermal expansion uponheating, the openings in the interconnector plate are closed in agas-tight manner by the contact elements. There are however also otherprocesses conceivable by means of which the openings in theinterconnector plate can be closed in a gas-tight manner by the contactelements, including for example soldering processes. With such a processit is conceivable to form, in a feature of the invention, a fuel cell orfuel cell stack (claim 9).

[0020] In the following, the invention is explained based upon thedescription of an embodiment with reference to the accompanying Figure.

[0021]FIG. 1 shows schematically a cross section through two fuel cells10 each comprised of an anode 5, a cathode 7 and an electrolyte 9. Thefuel cells 10 are connected together by the device 1. The device 1 iscomprised of an interconnector plate 2 in which openings and so-calledcontact elements 3 are incorporated. The openings are closed in agas-tight manner with the contact elements 3.

[0022] In FIG. 1 five contact elements 3 have been illustrated. Theinterconnector plate 2 is fabricated in this case from an iron-chromium,aluminum alloy with 5% of an aluminum component and has a thickness of200 μm. All of the set heads project uniformly above and below ehinterconnector plate 2 by one mm. The set heads 3 contain at least99.97% silver (pure silver). The spacing between the openings is uniformand about 1 cm over the entire area of the interconnector plate 2. Thesilver set heads 3 afford the advantage that at a use temperature of800° C. and both under reducing conditions and also oxidizing conditionsthey conduct the current well with long term stability. The evaporationrate of the silver set heads 3 is held low because of the small surfacearea exposed to the gas flow. A further advantage is obtained from thefact that the height of the silver set head 3 projects from theinterconnector plate 2 is 0.9 mm in that with this embodiment, noadditional gas distributor structure 4 is required. Additionaltime-intensive and gas-intensive working steps like milling or deepdrawing of components for producing the gas passages are thus avoided.In FIG. 1 a total of four gas distributor structures above theinterconnector plate and four gas distributor structures below theinterconnector plate have been indicated, of which, to save space, onlyone has been designated with a reference character. With the anode 5 ofthe fuel cell 10 shown as the lower fuel cell in FIG. 1, the contactwith the silver set heads 3 is ensured by means of an elastic nickelmesh 6. The elastic silver mesh 6 has a thickness of 250 μm and a meshwidth of 200 μm. The diameter of the wires amounts to 125 μm. For thecathode 7 of the fuel cell 10 shown as the upper fuel cell in FIG. 1,the contact with the silver set head 3 is ensured by means of an elasticsilver mesh 8. The elastic silver mesh has a thickness of 0.7 mm and amesh width of 0.9 mm. Without limitation of the invention, however,other values can be selected.

[0023] Further variants as to the configuration of the gas distributingstructures are also possible with respect to the shape and spacing ofthe contact elements in the interconnector plate. It is for exampleconceivable that, as in the bipolar plates, the contact elements can beconfigured as throughgoing ribs which separate the gas passages form oneanother to supply the electrodes, for example such that rectangularcontact elements close corresponding openings in a gas-tight manner.

1. A device (1) for electrically contacting electrodes (5, 7) of ahigh-temperature fuel cell, characterized by an interconnector plate (2)with openings and contact elements (3) therein for electricallycontacting the electrodes (5, 7) whereby the contact elements (3) onboth sides of the interconnector plate (2) project to a predeterminedheight and close the openings in a gas-tight manner.
 2. The deviceaccording to the preceding claim whereby all contact elements (3)project from the interconnector plate (2) to the same height.
 3. Thedevice (1) according to claim 1 or 2, characterized in that the contactelements (3) contain at least 99.97% silver.
 4. A high temperature fuelcell encompassing a device according to one of the preceding claims. 5.A high temperature fuel cell according to claim 4, encompassing anelastic nickel mesh (6) between the contact elements (3) and an anode(5) as an electrode.
 6. A high temperature fuel cell according to one ofclaims 4 or 5, encompassing an elastic silver mesh (8) between thecontact elements (3) and a cathode (7) as an electrode.
 7. A fuel cellstack encompassing at least two high-temperature fuel cells according toone of the claims 4-6.
 8. A method of making a device according to oneof claims 1-3, characterized by the steps: openings corresponding to abase area of the contact elements (3) are formed in an interconnectorplate (2), the contact elements (3) are inserted form-fittingly in theopening, the contact elements (3) are so connected that theinterconnector plate (2) that the openings are closed in a gas-tightmanner.
 9. A method of making a fuel cell or a fuel cell stack accordingto one of the claims 4-7 by the method according to claim 8.